Facet arthroplasty devices and methods

ABSTRACT

A prosthesis replaces all or a portion of a cephalad portion of a natural facet joint (i.e., an inferior articular process) on a vertebral body. The prosthesis has a prosthesis body that accommodates fixation to the vertebral body at or near a pedicle and without support by a lamina. The prosthesis also has an artificial facet joint structure carried by the prosthesis body adapted and configured to replace all or a portion of a cephalad portion of a natural facet joint.

RELATED APPLICATION

This application is a divisional of copending U.S. patent applicationSer. No. 10/615,727, filed Jul. 9, 2003, which is a divisional of U.S.patent application Ser. No. 09/693,272 (now U.S. Pat. No. 6,610,091),which claims the benefit of U.S. Provisional Patent Application Ser. No.60/160,891, filed Oct. 22, 1999, which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention generally relates to devices and surgical methodsfor the treatment of various types of spinal pathologies. Morespecifically, the present invention is directed to several differenttypes of spinal joint replacement prostheses, surgical procedures forperforming spinal joint replacements, and surgical instruments which maybe used to perform the surgical procedures.

BACKGROUND OF THE INVENTION

Back pain is a common human ailment. In fact, approximately 50% ofpersons who are over 60 years old suffer from lower back pain. Althoughmany incidences of back pain are due to sprains or muscle strains whichtend to be self-limited, some back pain is the result of more chronicfibromuscular, osteoarthritic, or ankylosing spondolytic processes ofthe lumbosacral area. Particularly in the population of over 50 yearolds, and most commonly in women, degenerative spine diseases such asdegenerative spondylolisthesis and spinal stenosis occurs in a highpercentage of the population. Iida, et al, 1989.

Degenerative changes of the adult spine have traditionally beendetermined to be the result of the interrelationship of the three jointcomplex; the disk and the two facet joints. Degenerative changes in thedisc lead to arthritic changes in the facet joint and vice versa. SeeFarfan and Sullivan, 1967; see also Farfan, 1969; see also Farfan, 1980.

One cadaver study of 19 cadavers with degenerative spondylolisthesisshowed that facet degeneration was more advanced than disc degenerationin all but two cases. Farfan. In mild spondylolisthetic cases, the slipappeared to be primarily the result of predominantly unilateral facetsubluxation. Other studies into degenerative changes of the spine haverevealed extensive contribution of facet joint degeneration todegenerative spinal pathologies such as degenerative spondylolisthesis,central and lateral stenosis, degenerative scoliosis, andkypho-scoliosis, at all levels of the lumbar spine. See Kirkaldy-Williset al, 1978; see also Rosenberg, 1975.

It has been determined that facet joint degeneration particularlycontributes to degenerative spinal pathologies in levels of the lumbarspine with sagittally oriented facet joints, i.e. the L4-L5 level.

When intractable pain or other neurologic involvement results from adultdegenerative spine diseases, such as the ones described above, surgicalprocedures may become necessary. Traditionally, the surgical managementof disease such as spinal stenosis consisted of decompressivelaminectomy alone. Herkowitz, et al, The Diagnosis and Management ofDegenerative Lumber Spondylolisthesis, 1998. Wide decompressivelaminectomies remove the entire lamina, and the marginal osteophytesaround the facet joint. Because a lot of degenerative spine disease hasbeen demonstrated to be caused by facet joint degeneration or disease,this procedure removes unnecessary bone from the lamina and insufficientbone from the facet joint.

Furthermore, although patients with one or two levels of spinal stenosistend to do reasonably well with just a one to two level widedecompressive laminectomy, patients whose spinal stenosis is associatedwith degenerative spondylolisthesis have not seen good results.Lombardi, 1985. Some studies reported a 65% increase in degree ofspondylolisthesis in patients treated with wide decompressivelaminectomy. See Johnson et al; see also White and Wiltse. The increasein spinal slippage especially increased in patients treated with threeor more levels of decompression, particularly in patients with radicallaminectomies where all of the facet joints were removed.

To reduce the occurrence of increased spondylolisthesis resulting fromdecompressive laminectomy, surgeons have been combining laminectomies,particularly in patients with three or more levels of decompression,with multi-level arthrodesis. Although patients who undergo concomitantarthrodesis do demonstrate a significantly better outcome with lesschance of further vertebral slippage after laminectomy, arthrodesisposes problems of its own. Aside from the occurrence of furtherspondylolisthesis in some patients, additional effects includenon-unions, slow rate of fusion even with autografts, and significantmorbidity at the graft donor site. Furthermore, even if the fusion issuccessful, joint motion is totally eliminated at the fusion site,creating additional stress on healthy segments of the spine which canlead to disc degeneration, herniation, instability spondylolysis, andfacet joint arthritis in the healthy segments.

An alternative to spinal fusion has been the use of an invertebral discprosthesis. There are at least 56 artificial disc designs which havebeen patented or identified as being investigated. McMillin C. R. andSteffee A. D., 20th Annual Meeting of the Society for Biomaterials(abstract) (1994). Although different designs achieve different levelsof success with patients, disc replacement mainly helps patients withinjured or diseased discs; disc replacement does not address spinepathologies such as spondylolisthesis and spinal stenosis caused byfacet joint degeneration or disease.

SUMMARY OF THE INVENTION

There is a need in the field for prostheses and prosthetic systems toreplace injured and/or diseased facet joints, which cause, or are aresult of, various spinal diseases. There is also a need for surgicalmethods to install such prostheses. There is also a need for prosthesesand prosthetic systems to replace spinal fusion procedures.

The present invention overcomes the problems and disadvantagesassociated with current strategies and designs in various treatments foradult spine diseases. The present inventive spinal arthroplastic systemsavoid the problems of spine stiffness, increased loads on unfusedlevels, and predictable failure rates associated with spinalarthrodesis.

The present invention pertains to spinal prostheses designed to replacefacet joints and/or part of the lamina at virtually all spinal levelsincluding L1-L2, L2-L3, L3-L4, L4-L5, L5-S-1, T11-T12, and T12-L1.Various types of joint replacement prostheses are described for treatingdifferent types of spinal problems.

One aspect of the invention provides a prosthesis to replace all or aportion of a cephalad portion of a natural facet joint (i.e., aninferior articular process) on a vertebral body. The prosthesiscomprises a prosthesis body that accommodates fixation to the vertebralbody at or near a pedicle and without support by a lamina. Theprosthesis also includes an artificial facet joint structure carried bythe prosthesis body adapted and configured to replace all or a portionof a cephalad portion of a natural facet joint.

Another aspect of the invention provides a method of replacing, on avertebral body, all or a portion of a cephalad portion of a naturalfacet joint (i.e., an inferior articular process) using the prosthesisdescribed in the preceding paragraph, to provide improved support forthe spinal column. The method comprises the steps of removing all or aportion of a natural articular process of the cephalad portion of thenatural facet joint from the vertebral body, and fixing the prosthesisto the vertebral body to replace the removed portion of the naturalarticular process with the artificial facet joint structure.

Features and advantages of the inventions are set forth in the followingDescription and Drawings, as well as in the appended Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lateral view of a spine with degenerative spondylolisthesisat L4-L5;

FIG. 2 is a front view of a universal facet replacement prosthesis;

FIGS. 2A, 2B, and 2C are view of an alternative embodiment of auniversal facet replacement prosthesis;

FIG. 3 is a lateral view of a spine with a superior universal facetprosthesis installed in a L5 vertebra;

FIG. 4 is a superior view of a L5 vertebra with an installed superioruniversal facet prosthesis;

FIG. 5 is a superior view of a L5 vertebra depicting removal of theprominent bone of the superior articular process;

FIG. 6 is a diagram illustrating the trimming of the superior facet todecompress a nerve root prior to reaming;

FIG. 7 is a superior view of a L5 vertebra depicting the reaming of thefacet into the pedicle;

FIG. 8 is a front view of a facet reamer;

FIG. 9 is a superior view of a vertebral body depicting broaching anopening into a vertebral body;

FIG. 10 is a superior view of a vertebral body depicting two universalfacet prostheses which have been installed in a vertebral body to formtwo superior facets;

FIG. 11 is a posterior view of a spine depicting an installed inferiorlamina/facet prosthesis;

FIG. 12 is a superior view of a vertebral body depicting completeprosthetic facet joints comprising an inferior lamina/facet prosthesisand two superior universal facet prostheses;

FIG. 13 is a lateral view of an installed complete prosthetic facetjoint;

FIG. 14 is a superior view of a vertebral body depicting sagittallyoriented arthritic facets with lateral stenosis;

FIG. 15 is a superior view of a vertebral body depicting removal of theinferior one eighth of the spinous process;

FIG. 16 is a superior view of a vertebral body after an inferiorlamina/facet resection;

FIG. 17 is a posterior view of a spine at an L4-L5 showing a spinousprocess resection line and inferior facet resection line;

FIG. 18 is a posterior view of an L4-L5 after part of the lamina andinferior facets have been removed, showing an installed universal facetprosthesis;

FIG. 19 is a posterior view of an L4-L5 after part of the lamina andinferior facets have been removed with an alternative V-type laminalcut, showing an installed universal facet prosthesis;

FIG. 20 is a posterior view of a L4 vertebra with an alternative shapedinferior lamina/facet prosthesis installed over a V-type laminal cut;

FIG. 21 is a posterior view of one embodiment of an installedhemi-lamina/facet prosthesis of the present invention;

FIG. 22 is a front view of one embodiment of a hemi-lamina/facetprosthesis of the present invention;

FIG. 23 is a posterior view of a spine, at an L4-L5 joint which hasundergone hemiarthroplasty with wide decompressive laminectomy, with twobase members of a hemi-lamina/facet prosthesis in the process of beinginstalled onto the L4-L5;

FIG. 24 is a posterior view of one embodiment of an installedhemi-lamina/facet prosthesis of the present invention;

FIG. 25 is a posterior view of one embodiment of an installedhemi-lamina/facet prosthesis of the present invention;

FIG. 26 is a posterior view of the L4-L5 depicting various cuts whichmay be made into the lamina a facets for a hemiarthroplasty with orwithout wide decompressive laminectomy;

FIG. 27 is a lateral view of the L4 and L5 vertebrae;

FIG. 28 is a superior view of the L4 and L5 vertebrae in a separatedcondition;

FIG. 29 is a front elevation view of a single-side prosthesis thatembodies the feature of the invention;

FIG. 30 is a side elevation view of the prosthesis shown in FIG. 29;

FIG. 31 is a lateral view of the L3, L4, and L5 vertebrae, with theprosthesis shown in FIG. 29 secured to the L4 vertebral body;

FIG. 32 is a lateral view of the L3 and L4 vertebrae, with a linksecured to the L4 vertebral body;

FIG. 33 is a lateral view of the L3 and L4 vertebrae, with a linksecured to the L4 vertebral body;

FIG. 34 is a front elevation view of another single-side facetprosthesis that embodies the feature of the invention;

FIG. 35 is a lateral view of the L3 and L4 vertebrae, with theprosthesis shown in FIG. 34 secured to the L4 vertebral body;

FIG. 36 is a front elevation view of a double-side facet joint linkassembly that embodies the feature of the invention, being formed of twocriss-crossing, mating link bodies;

FIGS. 37 and 38 are front elevation views of the link bodies forming thejoint link assembly shown in FIG. 36, being shown in a mutuallyseparated condition;

FIG. 39 is a front elevation view of an alternative embodiment of a linkbody that, when assembled with a mating link body, forms a joint linkassembly like that shown in FIG. 36;

FIG. 40 is a front elevation view of the double-side facet joint linkassembly shown in FIG. 36 in relation to its location on a vertebralbody;

FIG. 41 is a side view of a prosthesis, like that shown in FIGS. 29, 34,or 36, secured for use on the pedicle of a vertebral body (shown inlateral view); and

FIG. 42 is a side view of the lower end of the prosthesis shown in FIG.41, forming the inferior half of a facet joint, the superior half of thefacet joint being formed by a superior universal facet prosthesis shownin FIG. 2.

The invention may be embodied in several forms without departing fromits spirit or essential characteristics. The scope of the invention isdefined in the appended claims, rather than in the specific descriptionpreceding them. All embodiments that fall within the meaning and rangeof equivalency of the claims are therefore intended to be embraced bythe claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

I. Anatomy of Lumbar Vertebrae

FIGS. 27 and 28 show the fourth and fifth lumbar vertebrae L4 and L5,respectively, in a lateral view (while in anatomic association) and in asuperior view (separately). The lumbar vertebrae (of which there are atotal of five) are in the lower back, also called the “small of theback.”

As is typical with vertebrae, the vertebrae L4 and L5 are separated byan intervertebral disk 25. The configuration of the vertebrae L4 and L5differ somewhat, but each (like vertebrae in general) includes avertebral body 10, which is the anterior, massive part of bone thatgives strength to the vertebral column and supports body weight. Thevertebral arch 12 is posterior to the vertebral body 10 and is formed bythe right and left pedicles 14 and lamina 16. The pedicles 14 are short,stout processes that join the vertebral arch 12 to the vertebral body10. The pedicles 14 project posteriorly to meet two broad flat plates ofbone, called the lamina 16.

Seven other processes arise from the vertebral arch. Three processes—thespinous process 18 and two transverse 20 processes—project from thevertebral arch 12 and afford attachments for back muscles, forminglevers that help the muscles move the vertebrae. The remaining fourprocesses, called articular processes, project superiorly from thevertebral arch (and are thus called the superior articular processes 22)and inferiorly from the vertebral arch (and are thus called the inferiorarticular processes 24). The superior and inferior articular processes22 and 24 are in opposition with corresponding opposite processes ofvertebrae superior and inferior adjacent to them, forming joints, calledzygapophysial joints or, in short hand, the facet joints or facets. Thefacet joints permit gliding movement between the vertebrae L4 and L5.Facet joints are found between adjacent superior and inferior articularprocesses along the spinal column.

The facet joints can deteriorate or otherwise become injured ordiseased, causing lack of support for the spinal column, pain, and/ordifficulty in movement.

As described in this Specification, a facet joint has a superior halfand an inferior half. The superior half of the joint is formed by thevertebral level below the joint, and the inferior half of the joint isformed by the vertebral level above the joint. For example, in the L4-L5facet joint, the superior half of the joint is formed by structure onthe L-5 vertebra, and the inferior half of the joint is formed bystructure on the L-4 vertebra.

II. Superior Universal Facet Prosthesis

A. Structure

A superior universal facet prosthesis 330 is shown in FIG. 1 thatembodies features of the invention. The prosthesis 330 is designated“superior” because it creates an artificial facet surface for thesuperior half of the facet joint. The artificial surface articulateswith the inferior half of the facet joint. The prosthesis 330 allows forthe replacement of injured, diseased and/or deteriorating componentsalong the superior half of facet joints, to provide improved support forthe spinal column.

The universal facet prosthesis 330 may be constructed and configured invarious ways. The universal facet prosthesis 330 may, e.g., comprise acup member 315. The cup member 315 itself may be made of variousmaterials commonly used in the prosthetic arts including, but notlimited to, polyethylene, rubber, titanium, titanium alloys, chromecobalt, surgical steel, or any other total joint replacement metaland/or ceramic, bony in-growth surface, sintered glass, artificial bone,any uncemented metal or ceramic surface, or a combination thereof. Thecup member 315 may also be any appropriate shape including, but notlimited to, rectangular, disc shaped, trough shaped, or cup shaped. Thecup member may be fixed or anchored directly to a vertebra withpoly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails,bolts, anchors, break-away anchors and/or wires to facilitate any futureremoval of the prosthesis, or a combination thereof, or any other meansknown in the art.

As shown in FIG. 2, the cup member 315 is made of any joint materialscommonly used in the prosthetic arts, including, but not limited to,metals, ceramics, titanium, titanium alloys, tantalum, chrome cobalt,surgical steel, bony in-growth surfaces, artificial bone, uncementedsurface metals or ceramics, or any combination thereof, preferablycovered with a bony in-growth surface.

In the illustrated embodiment, the cup member 315 is fixed to a stem310, e.g., pre-welded, or glued with a biocompatible adhesive, orremovably secured using a frictional Morse taper. If desired, the stem310 can incorporate one or more fins or ribs (not shown), extendingoutward from the stem 310, which desirably reduce and/or eliminaterotation of the stem 310 once positioned within the targeted bone. Inaddition, the stem 310 can be cannulated, if desired, to allow the useof guide pins during insertion of the stem, as is well known in the art.

The stem 310 may itself be made of any joint materials commonly used inthe prosthetic arts, including, but not limited to, metals, ceramics,titanium, titanium alloys, tantalum, chrome cobalt, surgical steel, bonyin-growth surfaces, artificial bone, uncemented surface metals orceramics, or a combination thereof. In a preferred embodiment, the stem310 is covered with a bony in-growth surface.

In the illustrated embodiment, the cup member 315 carries a surfacemember, which is made of a material, e.g. polyethylene, ceramic, ormetal, which provides glide and cushioning ability for any potentialcontacting components, such as the articular head members describedbelow. In one embodiment (see FIG. 2 b), the surface member 325 can beformed in a gently upwardly curving shape, similar in shape to acatcher's mitt. In another embodiment (see FIG. 2 c), the surface member325 is rectangular in shape with rounded corners. The cup member 315 issized to be larger than the articulating superior half of the facetjoint, to allow for motion of the joint.

The surface member 325 may be a separate component that is fixed to thecup member 315, e.g., with a biocompatible adhesive, screws, nails, orcomprise a formed part of the cup member 315. The surface member 325 mayalso be held into the cup member 315 with compressive forces or friction(e.g., using a Morse taper).

As shown in FIGS. 2 a and 2 b, the stem 310 a could alternately comprisea threaded portion, such as in a pedicle screw, with the head orpedestal 315 a incorporating a depression 316 a sized to accommodate ahexagonal driver or other surgical driving tool well know in the art. Inaddition, the prosthesis 320 a could incorporate a lower insert 321 asized to fit into the depression 316 a in the head 315 a. If desired,the insert 321 a could comprise a Morse taper. In this embodiment, thestem 310 a can be screwed into the bone, with the insert 321 apositioned or otherwise secure within the depression 316 a. The stem 310a could be placed by tapping without screwing. If revision surgery isrequired, or some other condition required removal of the prosthesis,the insert 321 a can be removed from the stem 310 a, and the stem 310 acan subsequently be removed from the bone.

As FIG. 2 a shows, the stem 310 a can also include an enlargedprojection or collar 311 a abutting the cup member 315 a. The collar 311a serves to prevent unintended ingress of the stem 310 a further intothe pedicle, beyond a desired distance.

FIG. 1 depicts a spondylolisthetic spine with slippage at the L4-L5joint between the L4 and L5 vertebrae. FIG. 3 and FIG. 4 depict auniversal facet prosthesis 330 which has been installed into an L5vertebra 105 to replace the inferior half 305 of a facet joint. In oneembodiment, the stem 310 of universal facet prosthesis 330 is fixed intothe L5 vertebra 105 with poly (methylmethacrylate) bone cement,hydroxyapatite, a ground bone composition, or a combination thereof. Inanother embodiment, both the stem 310 and the cup member 315 are fixedto a vertebra with stainless steel wire to provide addition stability.

The new support provided by a universal facet prosthesis 330 helpscorrect degenerative spine diseases such as spondylolisthesis, spinalstenosis, or any spine disease. As demonstrated by comparing FIG. 1showing a spondylolisthetic spine with slippage between the L4 vertebra100 and the L5 vertebra 105 with FIG. 3 where the diseased superior half305 of the facet joint has been replaced with a superior universal facetprosthesis 330 of the present invention, correcting spondylolisthesis atthe L4-L5 joint and preventing further spondylolisthesis. Similarly,where correction of scoliosis and/or kypho-scoliosis is desired, thesize and/or shape of the prosthesis may be chosen to re-orient theaffected level(s) of the spine.

The superior universal facet prosthesis 330 described above may be usedas a replacement for the superior half of one or more of facet joints atany facet joint at any level of the spine. In the preferred embodiment,the universal facet prosthesis 330 is used to replace the superior halfof one or more facet joints in one or more facet joints. The superiorfacet prosthesis 330 is designed such that it has the appropriatecephalad and caudad directions as well as the appropriate medial/lateralangulation for the given level of the spine where the implant occurs.

In further embodiments, one or more surfaces of a universal facetprosthesis 330 may be covered with various coatings such asantimicrobial, antithrombotic, and osteoinductive agents, or acombination thereof. See, e.g., U.S. Pat. No. 5,866,113, which isincorporated herein by reference. These agents may further be carried ina biodegradable carrier material with which the pores of the stem and/orcup member of certain embodiments may be impregnated. See, e.g., U.S.Pat. No. 5,947,893, which is also incorporated herein by reference.

In still further embodiments of the present invention, a universal facetprosthesis may be attached to strengthened or fortified bone. Vertebraemay be strengthened prior to or during fixation of the prostheses usingthe methods, e.g., described in U.S. Pat. No. 5,827,289, which isincorporated herein by reference. This type of bone strengthening isparticularly suggested for osteoporotic patients who wish to have facetreplacement.

B. Surgical Method for Facet Replacement Using the Superior UniversalFacet Prosthesis

A surgical procedure that embodies features of the invention replacesthe superior half of a facet joint with the superior universal facetprosthesis 330 described above. The surgical procedure comprisesexposing the spinous process, lamina, and facet joints at a desiredlevel of the spine using any method common to those of skill in themedical arts. The prominent bone 306 b (see FIG. 5) may then berongeured using any means common in the field. The superior facet 305may also be trimmed, as depicted in FIG. 6, to decompress the nerve root203. A reamer 400, or any other instrument that is useful for grindingor scraping bone, may be used to ream the facet 305 b into the pedicle304 b as depicted in FIG. 7 and FIG. 8.

In a preferred embodiment (see FIG. 9), an opening 407 is made into thevertebral body 107 with a broach 405. The universal facet prosthesis 330b is installed into the opening 407 made by the broach 405, as shown inFIG. 10. The opening 407 may be partly filled with bone cement,hydroxyapatite, or any bone adhesive before installation of theuniversal facet prosthesis 330 b.

In an alternative embodiment, the stem 310 of the superior universalfacet prosthesis 330 may be constructed in such a way that the superioruniversal facet prosthesis 330 can be directly screwed or tapped intothe vertebral body 107.

In another arrangement, the cup member 315 of the universal facet member330 may additionally be fixed to the vertebral body 107 with bonecement, hydroxyapatite, or any other biocompatible adhesive. In yetanother arrangement, a universal facet prosthesis without a stem 310 maybe attached to the vertebral body with poly(methylmethacrylate) bonecement, hydroxyapatite, screws, nails, bolts, anchors, break-awayanchors to facilitate later removal of the prosthesis, or a combinationthereof, or any other means known in the art.

In a further embodiment of the present invention, the universal facetprosthesis 330 may be fixed into strengthened or fortified bone.Vertebrae may be strengthened prior to or during fixation of theprosthesis using the methods described in U.S. Pat. No. 5,827,289, whichis incorporated herein by reference. This type of bone strengtheningprocedure is particularly suggested for osteoporotic patients who wishto have facet replacement surgery.

III. Inferior Lamina/Facet Prosthesis

A. Structure

An inferior lamina/facet prosthesis 500 that embodies features of theinvention is shown in FIG. 11. The prosthesis 500 is designated“inferior” because it creates an artificial facet surface for theinferior half of a facet joint. The artificial surface articulates withthe superior half of the facet joint. The prosthesis 330 allows for thereplacement of injured, diseased and/or deteriorating components alongthe inferior halves of facet joints to provide improved support for thespinal column.

The prosthesis 330 may span the distance from a region on one side of avertebra to a region of the other side of the vertebra. It can thusreplace both inferior halves of a facet joint.

FIG. 14 depicts a superior view of a vertebral body depicting sagitallyoriented arthritic facets with lateral stenosis, showing how the spinalprocess 631 presses forward onto the nerve roots 205 and 200. Theprosthesis 500 allows for replacement of diseased and deterioratinginferior regions of the vertebra and partial replacement of lamina (seeFIG. 12), which may be pressing on the spinal nerves, to relieve pain.The prosthesis 500 creates artificial facet surfaces for the inferiorhalf of facet joints in the spine, which provide improved support forthe spinal column.

As FIG. 12 shows, a superior universal facet prosthesis 330, asdescribed above, may also be installed to replace the superior halves ofthe facet joints and, with the inferior lamina/facet prosthesis 500replacing the inferior halves of the facet joints, forming a total facetreplacement system that can result in entire artificial facet jointsalong a length of the spinal column. Alternatively, just the inferiorhalf one or more facet joints, or just the superior half of one or morefacet joints, may be replaced. The inferior and/or superior halves offacet joints may be replaced on one side of a given vertebra(unilateral), on the both sides of a given vertebra (bilateral), or acombination of each along a length of the spinal column.

The inferior lamina/facet prosthesis 500 may be constructed in variousways. As shown in FIG. 11, the prosthesis 500 can comprise a base member505. The base member 505 may be made of any joint materials commonlyused in the prosthetic arts, including, but not limited to, metals,ceramics, titanium, titanium alloys, tantalum, chrome cobalt, surgicalsteel, bony in-growth surfaces, artificial bone, uncemented surfacemetals or ceramics, or a combination thereof. The base member 505 mayalso be any appropriate shape to give appropriate support to the spineand to appropriately and sturdily attach to the inferior portions of avertebral body. The base member 505 may be fixed or anchored directly tothe inferior portion of a vertebral body with poly(methylmethacrylate)bone cement, hydroxyapatite, screws, nails, bolts, anchors, break-awayscrews to facilitate any future removal of the prosthesis, or acombination thereof, or any other means known in the art.

In a preferred arrangement, as depicted in FIG. 11, FIG. 12, and FIG.13, the base member 505 of the inferior lamina/facet prosthesis 500 isattached to each pedicle 102 a and 102 b with bilateral pedicle screws520 a and 520 b. The base member 505 of the inferior lamina/facetprosthesis 500 may further be attached to the spinous process 630 with atrans-spinous-process screw 515 to provide additional stability.

In another embodiment, the inferior lamina/facet prosthesis 500 may havea head member 510 for articulation with the cup member 315 of a superioruniversal facet prosthesis 330 or with a superior articular process ofthe adjoining vertebral body. The head member 510 may be made of variousmaterials commonly used in the prosthetic arts including, but notlimited to, polyethylene, rubber, tantalum, titanium, chrome cobalt,surgical steel, bony in-growth surfaces, ceramics, artificial bone, or acombination thereof. The head member 510 may further be any shape whichfacilitates attachment to the rest of the inferior lamina/facetprosthesis 500 and to smooth connection to, and movement in orientationto, a universal facet prosthesis 330 or a superior articular process ofan adjoining vertebral body. In one embodiment, a head member 510 isattached to the base member 505 of the inferior facet/lamina prosthesis500 with poly(methylmethacrylate) bone cement, hydroxyapatite, screws,nails, bolts, anchors, or any other means known in the art. The headmember 510 may also be removably attached by frictional engagement(e.g., using a Morse taper).

In a preferred embodiment (see FIGS. 11 and 12), the inferiorfacet/lamina prosthesis 500 comprises two head members 510 a and 510 bformed in the shape of an articular head. The head members 510 a and 510b preferably each have a Morse taper 512 at their upper surface to allowthem to lock into the base member 505 of the inferior facet/laminaprosthesis 500. Of course, either or both head members 510 a and 510 bcould be formed integrally with the prosthesis 500. In the preferredarrangement, a complete prosthetic facet joint 560 is provided (see FIG.11), in which the head members 510 a and 510 b articulate with the cupmember 315 of the superior universal facet prosthesis 330.

In further embodiments, one or more surfaces of the inferiorlamina/facet prosthesis 500 may be covered with various coatings such asantimicrobial, antithrombotic, and osteoinductive agents, or acombination thereof. See, e.g., U.S. Pat. No. 5,866,113, which isincorporated herein by reference. These agents may further be carried ina biodegradable carrier material with which the pores of the base memberand/or any screws, bolts, or nails of certain embodiments may beimpregnated. See, e.g., U.S. Pat. No. 5,947,893, which is incorporatedherein by reference.

In other arrangements, an inferior lamina/facet prosthesis 500 may beattached to strengthened or fortified bone. Vertebrae may bestrengthened prior to or during fixation of the prosthesis using themethods described, e.g., in U.S. Pat. No. 5,827,289, which isincorporated herein by reference. This type of bone strengthening isparticularly suggested for osteoporotic patients who wish to have facetreplacement.

B. Surgical Method for Partial Inferior Lamina/Facet Replacement Usingthe Inferior Lamina/Facet Prosthesis

A surgical procedure that embodies features of the invention replacesinferior lamina and articulated processes with the inferior lamina/facetprosthesis 500 as described above. The surgical procedure exposes thespinous process, lamina, and facet joints at a desired level of thespine using any method common to those of skill in the medical arts. AsFIG. 15 shows, an inferior one eighth to one half of the spinous process302 may be cut along the spinous process resection line 610 and removed,if the spinous process appears diseased or damaged. The cutting andremoval of the spinous process may be performed using any means commonin the field.

As shown in FIGS. 16 and 17, the inferior half of the facet joint mayalso be cut at or near the inferior facet resection line 600. In apreferred embodiment (see FIGS. 16 and 17), most of the lamina 615 ispreserved, as is the facet joint capsule 625, which may be opened andfolded back. In a preferred embodiment, the facet joint capsule 625 maybe cut perpendicular to its direction. The inferior half 621 of thefacet joint 620 may then be retracted from the superior half 622. Oncethe facet joint 620 is separated, the cut inferior bone 615 of the upperjoint (i.e. the cut inferior portion of the L4 vertebra in the L4-L5joint) may be removed. Alternatively, it may be possible to remove thecut inferior bone 615 while simultaneously separating the facet joint620.

In a preferred embodiment (see FIGS. 18 and 19), a superior universalfacet prosthesis 330 is then installed as previously described.Alternatively, the superior universal facet prosthesis 330 may beinstalled before the inferior bone is removed or even cut.

An inferior lamina/facet prosthesis 500 as described above may be placedonto the facet joints and over the spinous process. The inferiorlamina/facet prosthesis 500 may be fixed or anchored to the vertebralbody with poly(methylmethacrylate) bone cement, hydroxyapatite, screws,nails, bolts, anchors, break-away screws, or a combination thereof tofacilitate any future removal of the prosthesis, or any other meansknown in the art. In the preferred embodiment (see FIG. 11, FIG. 12, andFIG. 13), the inferior lamina/facet prosthesis 500 is attached to eachpedicle 102 a and 102 b of the inferior facets with bilateral pediclescrews 520 a and 520 b and is further attached to the spinous process630 with a trans-spinous-process screw 515 to provide additionalstability.

A head member 510 of an inferior lamina/facet prosthesis 500 mayarticulated into the cup member 315 of the superior universal facetprosthesis 330, or into a inferior half of a facet joint if the inferiorhalf has not been replaced, to create a complete prosthetic facet joint.

In an alternative embodiment, as depicted by FIG. 19, the inferior facetresection line 610 may be a V-type cut. If a V-type cut is used, anappropriately shaped inferior lamina/facet prosthesis 550 should beused, such as depicted in FIG. 20. The inferior facet resection line mayalternatively be cut in other ways, which are apparent to one of skillin the art of orthopedic surgery and will require inferior lamina/facetprostheses of varying shapes to appropriately fit the cut vertebra.

In a further embodiment of the present invention, a universal facetprosthesis and/or an inferior lamina/facet prosthesis may be fixed intostrengthened or fortified bone. Vertebrae may be strengthened prior toor during fixation of the prosthesis using the methods described, e.g.,in U.S. Pat. No. 5,827,289, which is incorporated herein by reference.This type of bone strengthening procedure is particularly suggested forosteoporotic patients who wish to have facet replacement surgery.

IV. Hemi-Lamina/Facet Prosthesis

A. Structure

A hemi-lamina/facet prosthesis 700 that embodies features of theinvention (see FIG. 21) may be used to replace parts of a lamina andinferior processes, some or all which may have been removed in a primaryprocedural bone resection, (i.e. with or without wide decompressivelaminectomy). The hemi-lamina/facet prosthesis 700 may be designedsimilarly, or even identically, to the inferior lamina/facet prosthesis500 described above, depending on how much of the bone is removed.

The hemi-lamina/facet prosthesis 700 may be constructed in various ways.In one embodiment, hemi-lamina/facet prosthesis 700 may, e.g., comprisea base member 705. The base member 705 may be made of any jointmaterials commonly used in the prosthetic arts, including, but notlimited to, metals, ceramics, titanium, titanium alloys, tantalum,chrome cobalt, surgical steel, bony in-growth surfaces, artificial bone,uncemented surface metals or ceramics, or a combination thereof. Thebase member 705 may be any shape which gives appropriate support to thespine and can be appropriately attached to the bone of the remaininglamina. The base member 705 may be fixed or anchored directly to theinferior portion of a vertebral body with poly(methylmethacrylate) bonecement, hydroxyapatite, screws, nails, bolts, anchors, break-away screwsto facilitate any future removal of the prosthesis, a combinationthereof, or any other means known in the art.

In a preferred embodiment (see FIG. 21) of a prosthesis forhemiarthroplasty (depicted as cut line 800 and further described below)without decompressive laminectomy, the base member 705 of thehemi-lamina/facet prosthesis 700 is attached to superior pedicle 102 bwith a pedicle screw 720. In another preferred embodiment, the basemember 705 of the hemi-lamina/facet prosthesis 700 may further beattached to the spinous process 630 with a trans-spinous-process screw715 to provide additional stability.

In a preferred embodiment (see FIGS. 24 and 25) of a prosthesis forhemiarthroplasty with wide decompressive laminectomy, thehemi-lamina/facet prosthesis 700 comprises at least one base member 705.The base member 705 may further comprise a pedicle attachment hole 725through which a pedicle screw 720, or a nail, anchor, break-away anchor,bolt, or any other fastening means, may be installed to help secure thehemi-lamina/facet prosthesis 700 to the inferior pedicle. In thepreferred embodiment, the base member 705 may also have at least onelamina attachment hole, with two lamina attachment holes 741 and 742pictured in FIG. 22, to further secure the hemi-lamina/facet prosthesis700 to the remaining laminal bone with screws, nails, anchors,break-away anchors, bolts, or any other fastening means. Parts of thehemi-lamina/facet prosthesis 700 which overlap bone may be additionallyfixed with bone cement, or any biocompatible adhesive.

A hemi-lamina/facet prosthesis 700 may further comprise a connectionplate, similar to the connection plate 750 depicted in FIG. 24, toconnect two base members, i.e. 705 a and 705 b, together. The connectionplate 750 may be fixed to each base member 705 a and 705 b with abiocompatible adhesive, screws, nails, bolts, compressive force, acombination thereof, or any other means common to those of skill in theart. Alternatively, a hemi-lamina/facet prosthesis 700 may furthercomprise at least one stabilization bar, similar to the stabilizationbars 761 and 762 depicted in FIG. 25. A stabilization bar or bars may befixed to each base member 705 a and 705 b with a biocompatible adhesive,screws, nails, bolts, compressive force, a combination thereof, or anyother means common to those of skill in the art. A hemi-lamina/facetprosthesis 700 may have any type of bridging or stabilizing members, orno bridging members at all, and may be comprised of any number of basemembers to provide appropriate stability to the spine. The bridgingmembers may be made of any joint materials commonly used in theprosthetic arts, including, but not limited to, metals, ceramics,titanium, titanium alloys, tantalum, chrome cobalt, surgical steel, bonyin-growth surfaces, artificial bone, uncemented surface metals orceramics, or a combination thereof.

In another embodiment, a hemi-lamina/facet prosthesis 700 may have ahead member 710 for articulation with the cup member 315 of a superioruniversal facet prosthesis 330 or with the superior articular process ofan adjoining superior pedicle. The head member 710 may be made ofvarious materials commonly used in the prosthetic arts including, butnot limited to, polyethylene, rubber, titanium, chrome cobalt, surgicalsteel, bony in-growth sintering, sintered glass, artificial bone, or acombination thereof. The head member 710 may further be any shape whichallows it to attach to the rest of the hemi-lamina/facet prosthesis 700and to smoothly connect to, and move in orientation to, the universalfacet prosthesis 330 or superior articular facet of the adjoiningsuperior pedicle. In one embodiment, the head member 710 is attached tothe rest of the hemi-lamina/facet prosthesis withpoly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails,bolts, anchors, a combination thereof, or any other means known in theart. The head member 710 may be removably attached, using, e.g., a Morsetaper.

In a preferred embodiment, hemi-lamina/facet prosthesis 700 comprises ahead member 710 made in the shape of an articular head. The head member710 preferably has a Morse Taper at its upper surface to allow it tolock into hemi-lamina/facet prosthesis 700.

In further embodiments, one or more surfaces of a hemi-lamina/facetprosthesis 700 may be covered with various coatings such asantimicrobial, antithrombotic, and osteoinductive agents, or acombination thereof. See, e.g., U.S. Pat. No. 5,866,113, which isincorporated herein by reference. These agents may further be carried ina biodegradable carrier material with which the pores of the base memberand/or any screws, bolts, or nails of certain embodiments may beimpregnated. See, e.g., U.S. Pat. No. 5,947,893, which is incorporatedherein by reference.

In still further embodiments of the present invention, ahemi-lamina/facet prosthesis 700 may be attached to strengthened orfortified bone. Vertebrae may be strengthened prior to or duringfixation of the prosthesis using the methods described, e.g., in U.S.Pat. No. 5,827,289, which is incorporated herein by reference. This typeof bone strengthening is particularly suggested for osteoporoticpatients who wish to have facet replacement.

B. Hemiarthroplasty With or Without Wide Decompressive Laminectomy Usingthe Hemi-Lamina/Facet Prosthesis

A surgical procedure that embodies features of the invention removes atleast part of a lamina and at least one superior portion of a facetjoint and replacing it with a hemi-lamina/facet prosthesis 700 asdescribed above. The general surgical procedure is generally similar tothe inferior lamina/facet replacement previously described, with themain difference being the types of cuts made into the laminal bone, andthat two separate prostheses are used to replace the superior portionsof two facet joints (left and right) of a given vertebra.

One embodiment of the surgical procedure comprises exposing the spinousprocess, lamina, and facet joints at a desired level of the spine usingany method common to those of skill in the medical arts. The inferiorfacet joint and part of the lamina may be cut with a hemiarthroplastyresection line 800 as depicted in FIG. 26 for a hemiarthroplasty. Thelamina may additionally be cut for a wide decompressive laminectomyalong the decompression resection line 810 as depicted in FIG. 26. Theinferior facet joint may be cut on one side or both sides of the lamina.Likewise, the lamina may be cut along a decompression resection line onone side or both sides.

In a preferred embodiment of a hemiarthroplasty without a widedecompressive laminectomy, leaving the cut inferior facet bone 300 inplace, the facet joint capsule 625 may be opened and folded back. In thepreferred embodiment, the facet joint capsule 625 may be cutperpendicular to its direction. The inferior half 621 of the facet joint620 may then be retracted from the superior half 622. Once the facetjoint 620 is separated, the cut inferior facet bone 825 may be removed.Alternatively, it may be possible to remove the cut inferior facet bone825 while simultaneously separating the facet joint 620.

In a preferred embodiment of a hemiarthroplasty with a widedecompressive laminectomy, a superior universal facet prosthesis 330 isthen installed as previously described, and depicted in FIG. 18.

A base member 705 of hemi-lamina/facet prosthesis 700 as described inany of the embodiments above may be placed onto at least one facet jointand at least one pedicle as depicted in FIG. 23, and over the spinousprocess if it has not been removed for hemiarthroplasty withoutdecompressive laminectomy as depicted in FIG. 21. The hemi- lamina/facetprosthesis 700 may be fixed or anchored to the vertebral body withpoly(methylmethacrylate) bone cement, hydroxyapatite, screws, nails,bolts, anchors, break-away screws to facilitate any possible futureremoval of the prosthesis, a combination thereof, or any other meansknown in the art. In the preferred embodiment, as depicted in FIG. 21,FIG. 24, and FIG. 25, the hemi-lamina/facet prosthesis 500 is attachedto each pedicle with bilateral pedicle screws 720.

A hemi-lamina/facet prosthesis 700 that may be used in hemiathroplastywithout wide decompressive laminectomy, depicted in FIG. 21, may furtherbe attached to the spinous process 630 with a trans-spinous-processscrew 715 to provide additional stability. A hemi-lamina prosthesis 700that may be used in hemiathroplasty with wide decompressive laminectomy,as depicted in FIGS. 23, 24, and 25, may further be attached toremaining laminal bone with screws, bolts, nails, anchors, or breakawayanchors through at least one lamina attachment hole 741 to provideadditional stability.

In embodiments where a hemi-lamina/facet prosthesis 700 with more thanone base member 705 is installed, a connection plate, depicted asconnection plate 750 in FIG. 24, at least one stabilization bar,depicted as stabilization bars 761 and 762 in FIG. 25, or any otherconnecting or stabilizing means known in the art, may be installed withthe base members to provide additional stability to the spine.

At least one head member, depicted as head member 710 in FIGS. 21, 23,24, and 25, of a hemi-lamina/facet prosthesis 700 may be articulatedinto a cup member of a superior universal facet prosthesis 330 to createa prosthetic facet joint capsule.

The embodiments may be used to replace one or more facet joints for theentire length of the spine from S1 to T11, on one side of a givenvertebra, or both sides of a given vertebra, or a combination thereofalong a length of the spine. If only one facet joint at a given level isto be replaced, the unilateral arthroplasty prosthesis for the inferiorhalf of the joint may be fixed to the superior ipso-lateral pedicle andinclude a box fitted over the spinous process, combined with screwfixation. The spinous process box is similar to the spinous process boxin the bilateral total facet arthroplasty embodiment previouslydiscussed.

In a further embodiment of the present invention, a universal facetprosthesis 330 and/or a hemi-lamina/facet prosthesis 700 may be fixedinto strengthened or fortified bone. The vertebrae may be strengthenedprior to or during fixation of the prosthesis using the methodsdescribed, e.g., in U.S. Pat. No. 5,827,289, which is incorporatedherein by reference. This type of bone strengthening procedure isparticularly suggested for osteoporotic patients who wish to have facetreplacement surgery.

V. Other Facet Prostheses

A. Single Side

FIGS. 29 and 30 show an inferior prosthesis 26 that embodies features ofthe invention. The prosthesis 26 is designated “inferior” because itcreates an artificial facet surface in the inferior half of a facetjoint. The artificial surface articulates with the superior half of thefacet joint. The prosthesis 26 is particularly well suited tosingle-sided procedures and/or for procedures involving vertebral bodieswhich are not symmetrical.

When the processes on one side of a vertebral body are differentlyspaced from those on the other side of the same body, the prostheses oneach side would desirably be of differing sizes as well. Moreover, it isoften difficult and/or impossible for a surgeon to determine the precisesize and/or shape necessary for a prosthesis until the surgical site hasactually been prepared for receiving the prosthesis. In such a case, thesurgeon typically needs a family of prostheses possessing differingsizes and/or shapes immediately available during the surgery. Thesurgeon cannot wait for a custom-fitted device to be created during thesurgery, so a number of prostheses of varying sizes and/or shapes mustbe available for each procedure.

The prosthesis 26 can be conveniently formed in different sizes andshapes, to offer an array of prostheses 26 from which the surgeon canpick and choose as surgery proceeds. This allows a surgeon to create a“custom” implant during the surgical procedure.

In the illustrated embodiment (see FIGS. 29 and 30), the prosthesis 26comprises a body 28 sized and shaped to span the distance between apedicle 14 and an inferior articular process 24 on the same side of avertebral body (see FIG. 31). The body 28 may be formed of a materialcommonly used in the prosthetic arts including, but not limited to,polyethylene, rubber, titanium, chrome cobalt, surgical steel, bonyin-growth sintering, sintered glass, artificial bone, or a combinationthereof.

The upper section of the body 28 desirably includes an opening 32. Theopening 32 accommodates a pedicle screw 34 (see FIG. 41), which securesthe upper end of the body 28 into the pedicle 14 of the vertebral body.The opening 32 could be elongated, to allow for varying orientationsand/or sizes of the pedicle screw 34. The remainder of the link body 28can be secured to the exterior of the vertebra using, e.g.,biocompatible adhesive.

The lower section of the body 28 is oriented to serve as the superiorhalf of a facet joint. The lower section of the body 28 desirablyincorporates a head 30. The head 30 can be permanently affixed to thebody 28, using, e.g., adhesive. Alternatively, the head can befrictionally secured, e.g., using a Morse taper, for removal andreplacement (as FIG. 41 shows). Like the body 28, the head 30 can beformed of a material commonly used in the prosthetic arts including, butnot limited to, polyethylene, rubber, titanium, chrome cobalt, surgicalsteel, bony in-growth sintering, sintered glass, artificial bone, or acombination thereof. The head 30 possesses a curvilinear shape thatdesirably curves along a gradual arc (as FIG. 42 shows), or can presenta “button” shape.

If desired, the lower section of the joint link body 28 could be angled,to more naturally mimic the orientation of a non-diseased facet joint.In one alternative embodiment, the lower section of the joint link body28 could rotate relative to the upper section, and could be rotationallysecured in a desired position by a surgeon using a locking screw orother locking means known in the art. Such an embodiment would allow thesurgeon to alter the orientation of the lower section to fit theparticular needs of a patient during the actual surgical procedure.

In use (see FIG. 31), the head 30 articulates with the superior half ofthe facet joint. The superior facet 22 can comprise the natural superiorarticular process itself (as FIG. 31 shows), or it can comprise asuperior prosthetic facet created, e.g., by the previously describeduniversal facet prosthesis 330 (as FIG. 42 shows) . The surface member320 of the universal facet prosthesis 330 can comprise a metal materialmade of, e.g., titanium, cobalt, chrome, etc., or a plastic materialsuch as, e.g., polyethylene, or a ceramic material. Thus the surgeon canselect the same or different materials to form the joint interfacebetween the head 30 and facet prosthesis 330.

FIGS. 34 and 35 show another embodiment of an inferior universalprosthesis 36 that embodies features of the invention. The prosthesis36, like the prosthesis 26, is designated “inferior” because it createsan artificial facet surface in the inferior half of the facet joint. Theartificial surface articulates with the superior half of the facetjoint. Like the prosthesis 26, the prosthesis 36 is particularly wellsuited to single-sided procedures and/or for procedures involvingvertebral bodies which are not symmetrical.

The prosthesis 36 comprises a body 38 sized and shaped to span thedistance between a pedicle 14 and an inferior articular process 24 (seeFIG. 35). The body 38 may be formed of the same types of material as thelink body 28. Like the link body 28, the upper section of the joint linkbody 38 desirably includes an opening 42, to accommodate a pedicle screw34 (see FIG. 35), which secures the upper end of the body 38 into thepedicle 14 of the vertebral body, in similar fashion as generally shownin FIG. 41. As before described with reference to the link 26, theopening 42 in the link body 38 could be elongated, to allow for varyingorientations and/or sizes of the pedicle screw 34. The remainder of thelink body 28 can be secured to the exterior of the vertebra using, e.g.,biocompatible adhesive.

Unlike the link body 28, the link body 38 includes an intermediateopening 44. In use (see FIG. 35), the spinous process 18 (if present)can extend through the opening 44, to stabilize the link body 38 on thevertebral body. Desirably, a trans-spinous-process screw 45 can be usedto provide additional stability.

The lower section of the joint link body 38 is oriented to serve as theinferior half of a facet joint. The lower section of the joint link body38 desirably incorporates a head 40, which can be constructed in thesame fashion as the head 30 of the link 26. Like the head 30, the facethead 40 can be permanently affixed to the body 38 or can be secured inwith a frictional fit (e.g., using a Morse taper) for removal andreplacement. Like the head 30, the head 40 can be formed of a materialcommonly used in the prosthetic arts.

In use (see FIG. 35), the head 40 articulates with the superior half ofthe facet joint with the next adjacent vertebra level. As beforeexplained for the link 26, the superior facet 22 can comprise thenatural superior articular facet 22 itself, or it can comprise aprosthetic facet created, e.g., by the previously described universalfacet prosthesis 330.

FIG. 32 shows a superior prosthetic link 26′ that also embodies featuresof the invention. The prosthetic link 26′ is designated “superior”because it creates an artificial facet surface in the superior half of afacet joint. The artificial surface articulates with the inferior halfof the facet joint. The superior prosthesis link 26′, like theprosthesis 26, is particularly well suited to single-sided proceduresand/or for procedures involving vertebral bodies which are notsymmetrical.

A stem 37 extends out from the upper end of the link 26′. The stem 37 isinserted (by screwing or tapping) into the pedicle, to thereby securethe link 26′ to the vertebral body.

As FIG. 32 shows, the upper end of the link 26′ is shaped to form a cup36, which articulates with the inferior half of the facet joint.

The inferior half of the facet joint can comprise the natural inferiorarticular process 24 itself (as FIG. 32 shows), or it can comprise thehead 30 of an inferior prosthesis 26 or link 26′ attached to the nextadjacent upper vertebra level (as FIG. 33 shows).

The lower end of the link 26′ can also carry a head 30 for articulationwith the superior half of a facet joint with the next adjacent lowervertebra. The superior half of the facet joint can comprise the naturalsuperior articular process 22 itself, or it can comprise the cup of alink 26′ attached to the next adjacent lower vertebra level.

It can thus be appreciated that the link 26′ is well suited for use inprocedures requiring replacement of multiple levels of facet joints, andcan be interlinked in superior and inferior pairs, like a structureformed out of interlinking tinker-toy pieces. The link 26′ also allowsubsequent surgeries to build upon already replaced levels, rather thanrequiring the removal and replacement of an existing implant toaccommodate replacement of failing facet joints in an adjacent level. Itshould be appreciated that the upper end of the prosthesis 36 can alsobe shaped to form a cup to articulate with the superior half of thefacet joint with the next adjacent upper vertebra level.

The prosthesis 26, 36, or link 26′ are well suited for use in a singleside of the vertebral body, such as where the facet joints need only bereplaced on a single side of the vertebral body. The prosthesis 26, 36,or link 26′ are also well suited for use in a dual-sided procedure wherethe vertebral body is either symmetrical or non-symmetrical. In thisarrangement, other prostheses 26, 36, or links 26′ can be secured on theopposite side of the vertebral body, allowing both sides of thevertebral body to be treated. Because the surgeon can pick prostheses26, 36, and links 26′ of varying sizes, depending upon the size of thevertebral site, and can individually position each prosthesis 26 or link26′ relative to the vertebral body, the surgeon can tailor the linkedimplant system to the individual's needs.

B. Multiple Level, Sequential Link Assemblies

FIG. 36 shows a universal prosthetic joint link assembly 56 thatembodies features of the invention. The joint link assembly 56 isparticularly well suited to double-sided procedures and for sequential,multiple level procedures.

In the illustrated embodiment (see FIG. 36), the joint link assembly 56comprises two criss-crossing link bodies 58 and 60. Each body 58 and 60(shown mutually separated in FIGS. 37 and 38, respectively) may beformed of a material commonly used in the prosthetic arts including, butnot limited to, polyethylene, rubber, titanium, chrome cobalt, surgicalsteel, bony in-growth sintering, sintered. glass, artificial bone, or acombination thereof.

As FIG. 36 shows, the link bodies 58 and 60 are desirably lockedtogether for use at an intermediate key-way 62, to form the x-shaped,criss-crossing assembly 56. The key-way 62 is formed by a shaped opening68 in one body 60 (see FIG. 37) and a mating shaped key 70 in the otherbody 58 (see FIG. 38). The key 70 nests within the opening 60 (as FIG.36 shows), to frictionally hold the bodies 58 and 60 together and resistrelative rotation between the bodies 58 and 60.

Of course, the shape of the opening 68 and key 70 can vary. In FIGS. 36,37, and 38, the opening 68 and key 70 are generally square orrectilinear in shape. In FIG. 39, an alternative link body 58 is shown,which possesses a key 70′ that is generally octagonal in shape, sized tonest within a corresponding octagonal opening in the other link (notshown). In this arrangement, the two link bodies 58 and 60 can bemutually assembled in different arcuately spaced orientations, allowingfor variations in facet joint size and positioning. If desired, thekey-way 62 could alternately be formed in a tooth and gear arrangement,which would desirably allow a multiplicity of potential arcuately spacedorientations for the two link bodies 58 and 60 forming the assembly 56.

The key 70 desirable peripherally defines an opening 72 (see FIG. 38),through which the spinous process 18 can (if present) project duringuse. This is generally shown in phantom lines by FIG. 41.

Alternatively, the link bodies 58 and 60 could be formed in acriss-crossing shape as a single, unitary body.

The upper section of each link body 58 and 60 desirably includes a cup64. The cups 64 form the left and right superior halves of a facet jointand, in use, articulate with the left and right inferior halves of thefacet joint.

A stem 65 extends out from the upper end of each link body 58 and 60.The stem 67 is inserted (by screwing or tapping) into the pedicle, tothereby secure the link bodies 58 and 60 to the vertebral body. In use,the stems 67 secure the upper end of the bodies 58 and 60 into anopposite pedicle 14 of a vertebral body.

As FIG. 40 best shows, the bodies 58 and 60 are each sized, shaped andmutually oriented to span the distance between a pedicle 14 on one sideof the vertebral body and the region of the inferior articular processon the opposite side of the vertebral body. The remainder of the linkbodies 58 and 60 can be secured to the exterior of the vertebra using,e.g., biocompatible adhesive. A trans-spinous-process screw 63 can alsobe used to provide additional stability.

The lower section of each link body 58 and 60 is oriented to serve asthe inferior half of a facet joint. As FIG. 40 shows, the link body 58,secured to the right pedicle, is positioned to serve as the inferiorhalf of the facet joint on the left side of the vertebra. The link body60, secured to the left pedicle, is positioned to serve as the inferiorhalf of the facet joint on the right side of the vertebra. For thispurpose, the lower section of each link body 58 and 60 desirablyincorporates a head 66. As before explained, the head 66 can bepermanently affixed to each body 58 and 60 or it can be secured in africtional way using, e.g., a Morse taper for removal and replacement.Like the bodies 58 and 60, the head 66 can be formed of a materialcommonly used in the prosthetic arts including, but not limited to,polyethylene, rubber, titanium, chrome cobalt, surgical steel, bonyin-growth sintering, sintered glass, artificial bone, or a combinationthereof.

In use, the heads 66 articulate with the superior halves of the left andright facet joints with the next adjacent vertebra level. As earlierdescribed with reference to the single link structures, the superiorhalves of the facet joints can comprise the natural superior articularprocess 22 itself, or it can comprise a prosthetic facet created, e.g.,by the cups 64 of another link assembly 56 secured to the next adjacentlower vertebra.

The interlocking of the crisscrossing link bodies 58 and 56 increasesthe strength of the overall link assembly 56. The link assembly 56distributes forces to both of the pedicles (and the spinous process, ifdesired), rather than relying upon fixation to a single pedicle.

Like the link 26′, the link assembly 56 is well suited for implantationin procedures requiring replacement of multiple levels of facet joints,and can be interlinked in superior and inferior pairs, like a structureformed out of interlinking tinker-toy pieces. Like the link 26′, thelink assembly 56 also allows subsequent surgeries to build upon alreadyreplaced levels, rather than requiring the removal and replacement of anexisting implant to accommodate replacement of failing facet joints inan adjacent level.

The size and shape of any prosthesis disclosed herein are desirablyselected by the physician, taking into account the morphology andgeometry of the site to be treated. The shape of the joint, the bonesand soft tissues involved, and the local structures that could be harmedif move inappropriately, are generally understood by medicalprofessionals using textbooks of human anatomy along with theirknowledge of the site and its disease and/or injury. The physician isalso desirably able to select the desired shape and size of theprosthesis and its placement in and/or around the joint based upon prioranalysis of the morphology of the targeted joint using, for example,plain film x-ray, fluoroscopic x-ray, or MRI or CT scanning. The shape,size and placement are desirably selected to optimize the strength andultimate bonding of the prosthesis to the surrounding bone and/or tissueof the joint.

Other embodiments and uses of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. All documents referenced herein arespecifically and entirely incorporated by reference. The specificationand examples should be considered exemplary only with the true scope andspirit of the invention indicated by the following claims. As will beeasily understood by those of ordinary skill in the art, variations andmodifications of each of the disclosed embodiments can be easily madewithin the scope of this invention as defined by the following claims.

1. A prosthesis to replace all or a portion of a cephalad portion of anatural facet joint on a vertebral body comprising a prosthesis bodyaccommodating fixation to the vertebral body at or near a pedicle andwithout support by a lamina, and an artificial facet joint structurecarried by the prosthesis body adapted and configured to replace all ora portion of a cephalad portion of a natural facet joint.
 2. Aprosthesis according to claim 1 wherein the artificial facet jointstructure is adapted and configured to replace a natural articularprocess of a cephalad portion of a natural facet joint.
 3. A prosthesisaccording to claim 1 wherein the artificial facet joint structure isadapted and configured to replace a natural articular process of acephalad portion of a natural facet joint after removal of at least someof a lamina from the vertebral body.
 4. A prosthesis according to claim1 wherein the artificial facet joint structure is adapted and configuredto replace a natural articular process of a cephalad portion of anatural facet joint after removal of at least part of an accessoryprocess from the vertebral body
 5. A prosthesis according to claim 1wherein the artificial facet joint structure is adapted and configuredto replace a natural articular process of a cephalad portion of anatural facet joint after removal of at least part of a transverseprocess from the vertebral body.
 6. A prosthesis according to claim 1wherein the artificial facet joint structure is adapted and configuredto replace a natural articular process of a cephalad portion of anatural facet joint after removal of at least part of a pedicle from thevertebral body.
 7. A prosthesis according to claim 1 wherein theartificial facet joint structure is adapted and configured to replace anatural articular process of a cephalad portion of a natural facet jointafter removal of at least some of the natural articular process from thevertebral body.
 8. A prosthesis according to claim 1 wherein theartificial facet joint structure is adapted and configured to replace anatural articular process of a cephalad portion of a natural facet jointafter removal of at least some of the natural articular process and ofat least some of a lamina from the vertebral body.
 9. A prosthesisaccording to claim 1 wherein the artificial facet joint structure isadapted and configured to replace a natural articular process of acephalad portion of a natural facet joint after removal of at least someof the natural articular process and of at least some of an accessoryprocess from the vertebral body.
 10. A prosthesis according to claim 1wherein the artificial facet joint structure is adapted and configuredto replace a natural articular process of a cephalad portion of anatural facet joint after removal of at least some of the naturalarticular process and of at least part of a transverse process from thevertebral body.
 11. A prosthesis according to claim 1 wherein theartificial facet joint structure is adapted and configured to replace anatural articular process of a cephalad portion of a natural facet jointafter removal of at least some of the natural articular process and ofat least part of a pedicle from the vertebral body.
 12. A prosthesisaccording to claim 1 wherein the artificial facet joint structure isadapted and configured to replace a natural articular process of acephalad portion of a natural facet joint, and wherein the prosthesisbody is adapted and configured to replace at least some of a lamina ofthe vertebral body.
 13. A prosthesis according to claim 1 wherein theartificial facet joint structure is adapted and configured to replace anatural articular process of a cephalad portion of a natural facetjoint, and wherein the prosthesis body is adapted and configured toreplace at least some of a mamillary process of the vertebral body. 14.A prosthesis according to claim 1 wherein the prosthesis body is adaptedand configured to replace at least some of a lamina of the vertebralbody.
 15. A prosthesis according to claim 1 wherein the prosthesis bodyis adapted and configured to replace at least some of a mamillaryprocess of the vertebral body.
 16. A prosthesis according to claim 1wherein the artificial facet joint structure is adapted and configuredto articulate with a caudal portion of a facet joint of an adjoiningvertebral body.
 17. A prosthesis according to claim 1 wherein theartificial facet joint structure is made of at least one selectedprosthetic material.
 18. A prosthesis according to claim 17 wherein theselected prosthetic material includes polyethylene, rubber, tantalum,titanium, chrome cobalt, surgical steel, bony in-growth material,ceramic, artificial bone, or a combination thereof.
 19. A method ofreplacing, on a vertebral body, all or a portion of a cephalad portionof a natural facet joint using the prosthesis defined in claim 1 toprovide improved support for the spinal column, the method comprisingthe steps of (i) removing all or a portion of a natural articularprocess of the cephalad portion of the natural facet joint from thevertebral body, and (ii) fixing the prosthesis as defined in claim 1 tothe vertebral body to replace the removed portion of the naturalarticular process with the artificial facet joint structure.
 20. Amethod according to claim 19 further including a step of removing atleast part of an accessory process from the vertebral body.
 21. A methodaccording to claim 19 further including a step of removing at least partof a transverse process from the vertebral body.
 22. A method accordingto claim 19 further including a step of removing at least part of apedicle from the vertebral body.